How to correctly use the mirror modifier?
It is indeed possible :).

1) Choose the right axis
There are three axis to mirror around.
Trial and error will get you there fast enough :).


2) Don't move the object, move the mesh.
The mirroring center is your object's origin point.
So instead of moving your object, switch to Edit Mode and move your mesh farther from the origin point.



3) Apply rotation
Object's local rotation can mess with modifiers.
Go to Object->Apply->Rotation.


Further Reading:
Mirror Modifier page in Blender Manual